I. What are pull current, sink current and absorption current?
Pull current
Pull current refers to the current that is actively supplied to the load when the output is in a high level state. When a digital circuit outputs a high level, the output will provide current to the external circuit, and this current is the pull current. The pull current reflects the chip's ability to drive when outputting a high level.

Potting Current
The potting current is the current that flows into the chip from the external circuit when the output is in a low level state. When a digital circuit outputs a low level, the current on the load will flow to the output, and this inflow of current is called the potting current. The potting current reflects the ability of the chip to withstand the output low level.

Absorbed Current
Absorbed current is the current that is actively drawn from the outside at the input of the chip. Unlike the passive inflow of the potting current, the absorbed current emphasizes that the input pins are actively absorbing the external current. Absorbed current is often used to characterize input ports.

II. Relationship and difference between pull current and sink current
Pull current and sink current are both for outputs, corresponding to high and low level states respectively.
Pull current is the flow of current from inside the chip to the outside, while sink current is the flow of current from outside into the chip.
When the pull current is too large, the output high level may become low; when the sink current is too large, the output low level may become high.
Therefore, the chip design must limit the maximum pull current and maximum irrigation current to ensure that the logic level is stable and reliable.

III. The significance of pull current, sink current and absorption current in chip specifications
The pull current and sink current labeled in the chip manual are the maximum values allowed by the chip.
In practice, the pull current and sink current of the output can not exceed the nominal maximum value of the chip, otherwise it will lead to abnormal output level or even damage the chip.
Absorption current indicator is mainly used to describe the load characteristics of the input port and the ability to absorb external signals.

IV. Basic Concepts of Pull-up Resistors and Pull-down Resistors
What is a pull-up resistor?
A pull-up resistor is a signal that is connected to the positive terminal of the power supply through a resistor. Its function is to ensure that the signal is high when it is hanging or not driven.
When the output is open or weakly driven, the pull-up resistor provides the necessary current path to stabilize the signal at a high level.

What is a Pull Down Resistor
A pull-down resistor is a signal that is connected to ground through a resistor. It serves to ensure that the signal is at a low level when it is dangling or not driven.
Pull-down resistors also play a role in preventing suspension, stabilizing the input state, and enhancing anti-interference.

V. Pull-up resistor and pull-down resistor role summary
1, generally used as a single-key trigger, if the IC itself does not have a built-in resistor, in order to make the single key to maintain in the state of not being triggered or triggered back to the original state, you must connect a resistor outside the IC.
2, digital circuits have three states: high level, low level, and high resistance state, some applications do not want to appear high resistance state, you can pull-up resistor or pull-down resistor to make in a stable state, depending on the design requirements!
3, generally speaking, the I / O port, some can be set, some can not be set, some are built-in, some need to be connected externally, the output of the I / O port is similar to a transistor C, when the C connected through a resistor and the power supply is connected together, the resistor becomes a pull-up resistor on the C, that is to say, if the port is normally a high level; C through a resistor and the ground connected together, the resistor is called a pull-down resistor, so that the port is normal; C through a resistor and ground connected together, the resistor is called a pull-down resistor, so that the port is normally a low level, the role of: for example: “When a port connected to a pull-up resistor is set to the input state, he is normally a high level for detecting low-level input”.
4, pull-up resistor is used to solve the bus drive capacity is insufficient to provide current. The general term is pull current, pull-down resistor is used to absorb the current, that is, we usually call the irrigation current
5, connected to the resistor is to prevent the input side of the overhang
6, attenuate the external current on the chip to produce interference
7、Protect the protection diode within the cmos, the general current is not greater than 10mA
8, through the pull-up or pull-down to increase or decrease the drive current
9, change the level of potential, commonly used in TTL-CMOS matching
10, in the pin suspension when there is a definite state
11, Increase the driving ability when high level output.
12、Provide current for OC gate

VI. Pull-up resistor resistance selection principles
1, From the power saving and the chip's ability to fill the current should be large enough to consider; large resistance, small current.
2, From ensuring sufficient drive current should be small enough to consider; small resistance, high current.
3, For high-speed circuits, too large a pull-up resistor may be flattened edges. Comprehensive consideration
The above three points, usually selected between 1k and 10k. Similar reasoning applies to pull-down resistors.
The selection of pull-up resistors and pull-down resistors should be “set in conjunction with the characteristics of the switching tube and the input characteristics of the lower-stage circuit, and the following factors should be considered:
1. The balance between driving ability and power consumption. Take pull-up resistor as an example, generally speaking, the smaller the pull-up resistor, the stronger the driving ability, but the higher the power consumption, the design should pay attention to the balance between the two. 2.
2. Driving requirements of lower-level circuits. Also take the pull-up resistor as an example, when the output level is high, the switching tube is disconnected, the pull-up resistor should be properly selected to be able to provide enough current to the lower level circuit. 3.
3. High and low level setting. Different circuits have different threshold levels for high and low levels, and the resistor should be set appropriately to ensure that the correct level is output. Take pull-up resistor as an example, when the output is low level, the switching tube conducts, the pull-up resistor and the switching tube conductive resistor divider value should be ensured to be under the zero level threshold.
4. Frequency characteristics. Take pull-up resistor as an example, the capacitance between the pull-up resistor and the drain-source stage of the switch and the input capacitance between the lower stage circuit will form “RC delay”, the larger the resistance, the larger the delay. The setting of pull-up resistor should consider the needs of the circuit in this regard.
The principle of pull-down resistor setting is the same as that of pull-up resistor.

VII. Impact of pull current and sink current on the performance of logic gates
When the logic gate outputs a high level, the higher the pull current, the lower the output voltage. It is necessary to ensure that the output high level is not lower than the minimum value specified in the specification, for example, the high level of TTL logic gate requires UOHMIN ≥ 2.4V.
When the logic gate outputs a low level, the higher the irrigation current, the higher the output voltage. It is necessary to ensure that the output low level is not higher than the highest value specified in the specification, for example, the low level of the TTL logic gate requires UOLMAX ≤ 0.4V.
Therefore, reasonable control of the pull current and the irrigation current is the key to ensure the normal operation of digital circuits.

VIII. Summarize
Pull current, sink current and absorption current are important parameters to measure the input-output capability of a chip.
Understanding these concepts helps us to select the chip correctly, design the circuit reasonably, and avoid abnormal levels or even device damage due to excessive current.
In digital circuit design, pull-up resistors and pull-down resistors are used reasonably to further improve the stability, anti-interference and reliability of the system.